Conventional cellular systems based on Frequency Division Multiple Access (FDMA) or Time Division Multiple Access (TDMA) were intended to avoid interferences. Since, in those conventional cellular systems, adjacent cells do not share common resources, it was possible to achieve a sufficient signal to interference ratio (SIR), yet a lower system capacity due to frequency reuse efficiency could not be inevitable. Since the conventional cellular systems based on FDMA or TDMA were mostly targeting voice services having a constant data rate, increasing a number of available channels having a sufficient SIR through electric power control corresponds to increasing a capacity in the conventional cellular systems based on FDMA or TDMA. From this point, a voice system based on Code Division Multiple Access (CDMA) which significantly increases efficiency in frequency reuse was spread, and through interference averaging and reducing a range of fluctuation per channel, that an acceptable level of interferences in a great number of channels was achieved. However, as the main intention of the service was changed from the voice service which has the constant data rate to a packet service which has a flexible data rate, maintaining the adequate interference through averaging interference was not always optimized. Also, as technology of Orthogonal Frequency Division Multiple/Orthogonal Frequency Division Multiple Access (OFDM/OFDMA) is developed for a cellular domain, which is adequate for avoiding interference, the interference issues with adjacent cells resurfaced.
In order to resolve the interference issue in the adjacent cells in the OFDM/OFDMA based cellular environment, an interface averaging using frequency hopping is frequently used. Using different hopping patterns, although not at the level of averaging within a symbol of CDMA, it is possible to achieve sufficient averaging within encoded packets.
In such interference averaging, there is a mechanism for partially applying interference avoidance of a frequency reuse concept to divide overall resources into a resource space and a secondary resource space. In this mechanism, traffic with similar properties are collected and allocated to a single resource space and a single secondary resource space. The conventional method introduced matching traffic having similar properties with each other in multi cell environments. In an example embodiment of the conventional method, all cells are divided into three frequency reuse patterns, and one resource space is divided into three secondary resource spaces, and each of three cells mainly uses one of the three secondary resource spaces and allows transmission with a relatively large electrical power and for the remaining two secondary resource spaces, a small, restricted amount of electrical power is permitted for the transmission to limit the interferences to the adjacent cells. With such an example embodiment of the conventional method, in actuality, when a user having inferior channel properties in a cell boundary communicates with a frequency reuse factor of 3 using ⅓ of overall resources, another user having superior channel properties may perform communications without affecting users around a base station.
In a frequency reuse method based on an identical concept of the above conventional method, users inside a cell operate with a frequency reuse factor of 1, and users in a cell boundary operate with a frequency reuse factor of 3. In overall resources, there are common resources shared by the users inside a cell, and the remaining resources are divided into three parts for users in the cell boundary, for each of the cells.
To simultaneously overcome the conventional issues of the users in the cell boundary and the issues of the efficiency in frequency reuse, a method for managing an inter-cell interference based on directivity and concentration of interferences in uplinks has been provided. Each terminal has a different level of interference affecting adjacent cells for each adjacent base station, and looks up a base station which the terminal is affected by the greatest interference. Using this property inversely, each of the base stations collects terminals with great interferences and receives the collected interferences at one time. This mechanism would seldom result in a great level of interference, yet it is overall beneficial since the interference level is significantly low in most cases. Also, the interference level is low as well when a terminal in the cell boundary is being serviced, thereby improving the performance.
The above method is for the uplink, and a method for a downlink also intends to a direction of utilizing great interference, here, benefit may be obtained from reducing an electrical power in base stations which gives great interference. Conversely, a terminal performs communication when electrical power in a base station with great interferences are reduced. The operations may differ, yet geographical dispositions of the terminals are largely similar, and this mechanism of downlinks is relatively simpler than the previous mechanism of uplinks.
However, even with the conventional fractional frequency reuse methods employed, irregular cell arrays and shadowing effects from the obstacles in a propagation route of radio waves result in users in the cell boundary experiencing the inter-cell interferences, and the above methods may not provide a solution to such issues.
Also, since the conventional fractional frequency reuse methods are intended for traffic channels or for controlling channels for specific users, such methods fail to resolve the issues of inter-cell interferences of downlink control channels which may be receivable by all terminals.